Time division multiplex system



Nov. 4, 1952 D. s. BOND 2,616,975

TIME DIVISION MULTIPLEX SYSTEM Filed Feb. e, 1947 :s sheets-sheet 2 IN VEN TOR.

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Patented Nov. 4, 1952 v TIME DIVISION MULTIPLEX SYSTEM Donald Spencer Bond, Philadelphia, 1?a.,. assignoi` tc Radio Corporation of America', a'corporat'ion i of Delaware v,Appliation February 6, 1947, Serial No. 726,876 e 'p12 Claims- 1 This invention relatesf generally .to i communication systems and more particularly to a multiplex vsystem in which it. is: desired to transmit a plurality of voice, telegraph,vor. other Signals simultaneously over a suitable wide frequency band transmission circuit; for-example, radio, Wire or other transmission line or Wave guide circuit.

An object of the invention is to provide a method of and means for efficiently effecting communication` v, over a A relay system employing one or more repeater stations spaced along the line of ltransmission between theitransmitting terminal and the .receivingrterminal and wherein stringent requirementsare imposed on signal-tonoise ratio, cross-modulationl harmonic distortion, and vsimilar characteristics. Y Another object is to. provide multpleXne means of. greater simplicity and lowerl cost than vthe conventional multiplex means whichy employ frequency-division apparatus utilizingk modulations and Wave-filters.

Ithave been demonstrated-that the lsignal-tonoise ratio (S/N) of a multi-channel system in which a series of 'amplitude-modulated pulses (PAM) directly ymodulates the frequency cf a radio-frequencycarrier wave bears .the .following relation to the signal-tofnoise :ratio for a Single channel amplitude-modulated system. In this relation, the Successive. steps of ,this first system are herein identified by .thesymbols PAM-FM, in which the symbol group PAM refers l.to the step of producing .A amplitude-modulated pulses .in which the amplitudes ,vary in accordance with vthe instantaneous samplesof the input wave, andthe symbol FM refers tothe stepof rfrequency modulating the radio-fnequencycarrier by theseamplitude-modulated pulses, Vwhile-r,thesynlbol group AM refers Vto a, single; channel ,amplitude-modulatedsystem.

where B is v.the totalband-width of the R.-F. spectrum of the complex wave constituting `the nal output Wave, n is the numbercf channels, and fm is the maximumaudio frequency of each channel. However, .theiPAM--F'Mysystem isinadequate in an important respect in relay systemshaving a considerable number ofspaced'repeater Stations arranged in .tandem because the intra-channel distortion increases progressively. due .to `the diniculty of demodulating, amplifyingandmodulating with ythe AMpulses in a sitrictlylinear fashion. Briefly stated, my invention providesa multiples System whiehie ehereterieeby high Signal-A toi-noise ratio1ow cross-modulation and low harmonic distortion and relatively Simple and inexpensive terminal equipment.'v Thisinverition has particular usefulness in,A systems employing a large number of spaced-repeater station in tandem. These advantages are achieved, in part, by causing the signalin v`each channel to produce a series of laniplitud'e-nfiorl'ulatedfDL-C. pulses (PAM), utilizing these p'uls'e's'to frequencymodulate (FM) a sub-carrier wave whose frequency is considerably higher than the maximum frequency of the Vsignal in the channel, and causing the resultant frequency-modulated sub-carrier Wave to frequency modulate (FM) a radio frequency ear'- rier of higher frequency thanthe :Sub-Lcarr-ier. Thest-eps in this rethod .of operationof the invention are herein identied `by` the symbols PAM-FM-FM where the symbol group PAM refers to pulse amplitudemodulation, ,and FM to frequency modulation. In-practicing the multiplex system of. the invention, eachcycle ofz operationsy comprises a slejie of pulses constituted by a pulse frein. each, channel sedue 1a11y folldwe'd "by individual pulses from the' other channels in Aa predetermined yerder- .Thu,s., each Cycle fefehprises a pulse frein, let usisaychannel #1, fnllowed by a pulse from'channel #2, then followed by a pulse from channel 5=3 etc., upto and ineludine .the cth ,01' letychelilelf The ,next eyele will repeeilthie sequentie ihiheseme order.r ,Eeeh cycle may include one" ormore syncljirp'nizing pulses to establish the reference time for leach cycle. At the receiving terminal, the foregoing steps are reversed to derive the-,frequency-modulated .sub-.carrien theeembined PAMsgnal. the Separate channelgroups of pulses, and nally the original signal for each channel. Atintermediate repeater or Y,relay stations, the frequency-modulated sub-carrier is derivedV froinyfthe received raquo-frequency` wave, ampli'ediwand the amplifled wave caused vto frequency modulate a radiofrequency transmitteroscillator.

Thefpresent invefrliti'dn` s ifices te a small., and unimportant .degree the b llentsigial-,t noise ratio of.. the` system, order lto obtain the more irnportant` Aadv ntages Loutlined aboveln regard to those charact'e ic wh fh' argessential to systems involving am r.` ,rep'eater'stations in tandem. The larger theniirnber` of frepeater stations employed the greater willb the degreeofimportanc'eQftheiinv n.' Thusfor a number'of repeater Astation, c' ding'S'tcfZO, the.invention.becomesofv .1

The liheorneiveehelew (2.5-1- 1) fd1=3.5fd1

Then for a nal modulation index of R2, the minimum R.F. bandwidth Brr will be Brf=7fd1R2 =3.5BseR2 when BSG is the sub-carrier band with and corresponds to B of Eq. 1.

. The signal-to-noise ratio for a xed band-width is modified by a factor Efe 2fsc where faz is the frequency deviation produced by the sub-carrier. The mean frequency of the sub-carrier is designated fsa. The modulation where Brf in Eq. 6 is the nal R.-F. band-width. Equation demonstrates the origin of the value of 14 db which is equivalent to the voltage ratio of one-fifth.

As an example, a comparison can be made of various modulation schemes for a system of 12 telephone channels, in each of which a 56-db signal-to-thermal-noise ratio is'to be obtained. The required received powers at threshold and required rf bandwidths are given in the following table:

Required Required power bandwidth The symbol SS used `in the above table refers to waves obtained, by amplitude modulation of a continuous carrier wave and suppression of said carrier and one of the resulting sidebands. The symbol PM refers to phase modulation, while the symbol PPM refers to pulse-phase modulation, sometimes known as pulse-position or pulse-time modulation.

It will be noted in the above table that the required power and bandwidth for system 2 are greater, respectively, than those of system 1 by the factor (5)2/3=2.92. See Landon, RCA Review, volume 9, pages 433-482, September, 1948. In this reference, the improvement by a factor 4 of 1.65 for cross-talk balancing is demonstrated.

When suitable weight is given to both the factors of required power and required bandwidth, it becomes evident from the practical point of View that the two PAM systems 1 and 2 in the above table are notably superior to the other time division systems, viz. systems 3 and 4 and also to system 6. For other practical and well-known considerations resulting from the use of time division methods instead of frequency division multiplex methods, systems 1 and 2 are to be preferred to system 5 of the above table.

Other objects, together with the different features of the invention, will appear from a reading 0f the following detailed description in conjunction with a drawing, wherein:

Fig. 1 diagrammatically illustrates a relay system having a plurality of repeater stations for effecting the objects of the invention;

` Fig. 2 diagrammatically illustrates in box form one embodiment of the transmitting terminal of the system of the invention;

Fig. 3 diagrammatically illustrates in box form one embodiment of the receiving terminal of the system of the invention;

Fig. 4 diagrammatically illustrates, in box form, one embodiment of a relay or repeater station which may be used in the transmission circuit between the transmitting and receiving terminals;

Fig. 5 illustrates details of the transmitting system of Fig. 1;

Figs. 6a. and 6b and 7 are curves given in explanation of the operation of the invention; and

Fig. 8 illustrates details of the receiving system of Fig. 2.

Fig. 1 shows diagrammatically a multiplex relay system to which the present invention may be applied. The terminal transmitting equipment is designated by box A and is supplied with separate audio signals from a plurality of different channels. The equipment in box A is illustrated in more detail in Figs. 2 and 5. The radio-frequency output from the terminal transmitter is radiated over directive antenna 155 toward the next repeater station B which has a directive receiving antenna 15| pointed toward the terminal transmitter A and a directive transmitting antenna 152 pointed toward the next repeater station C in the line of transmission. Repeater sta- The terminal receiving equipment is indicated by box D has a directive receiving antenna 153 pointed to receive radio signals from the nearest repeaterstation. This equipment is illustrated in more detail in Figs. 3 andv 8, and serves to demodulate the received signals and separate the different signals into their respective channels.

The transmittingterminal equipment of the invention, shown in Fig. 2, includes a multiplicity of signal channels labeled channel #1, channel #2, etc. up to channel #'n. There may, for example, be anywhere from 5to 50 or more separate channels. Each channel supplies its own input signal which may be speech, telegraph, or any other suitable audio input, and these signals are independent of each other. In circuit with each channel is a pulse generator 1, 2, or 3, etc. The pulse generators for the different channels have respectively been given the same reference numerals as the channels in which they are located. The outputs from the. channel pulse generators are combined in a single connection I3. A synchannel i .pulse Igeneratorsfandtheesynchrnnizing pulse generator arecenclosed Witliin-ardashedlinea boxzl i labeled PAM modulator; f

Each channel fpulse :.generatorsf producirse.s means not shown, pulses.. at .farepetitionvrate vfr the1 order:of.2.5.tof34 timeslthe" highestfaudd fre quency fm ofV the. input fsignalewaveforfthat:chan nel.: L. These. .pulses arefofnA aemagnitu'de "llpropor tionalxto iand of the'.same'lalgebraiclsignf'as .th modulating audio-frequency signal at the 'timeo .ai thenpulse generation; i. By' thewterniisamef algebraio signi is meant `thefpulsesiare -eithenspositiven or negative dependingeuponifthe 1 instantaneou i555 polarity of. theeinputwsignalawave Obviously; rif ..-;.f desired; .the system; can .i-bezso '.,rdesi-gned Athat th pulse may becofl.oppositemolari-ty toatheizpolarity ofv the: instantaneously; fapplie'd signallwave. l pulses from the" channelpulsefgeneratmlrtfortex.2 ample :.from pulse, generatorf;#l')l zrepresen A' samples of' the' input-.sgn'aL'WaVe :.at'timerelinter vals; l/frf; r The other: channel 'i pulse; gez'ierfators,4 for examplef. pulse ,generators 4t2;.:#3;fetciperex-A form thezsarne functiona asgeneratortlfat" suoi-1 251 cessively later epochs in .theatimecinterva l/f Thus, a series of .-n (number/mfxchannels'): ul'se appears at commonioutputfconnection 'I31clu`ringu` this interval as a result of this PAM processfsislndf addition;A one; or more;y characteristic.:synchrof 30. nizing. signals. arev generateds byisynchronizingw: pulse` generator l5 duringzitheasame Qintervalll/'Jli and added to the series of n pulsesialready. 'de-.2... scribed. ,and 1 in a .suitable.;.gapfin.',theppulsensee quence. The synchronizing@pulsess.:maycbe'i'dis,235

tinguished from the channel pulses .by duration@l amplitude, occurrence; in acloselyidspacedf pairs of?v the same or .alternate'fpolarty aoteristics.

The; original audio-frequencyminputu'signaliL40 Wave for channel 1.is showmbyztheisinefwave '.ofv.`

or :'bylother: char.;-v

. each; channel for yeach frameiy or.- cycleflibflifoperJ-r ations is a \D.C. amplitudeimodula`ted pulses"- whosey amplitude is proportional-.toandxofs then.: samegalgebraic sign: as the modulatingi audio-1 frequency f input Wave; forl'fthatn..ch'annelw-It should lbe 4noted that .thelsamplesaof i pulses from channel #l are of ;positive.rzpolarityl1f ats-time B and -,N, though o'f.;different:imagnitudeglzand of negative polarity. at time-Za ,1i-The--synchroniz`- ing pulse 1S occurs..once-..during eachfframeio cycle vof'. operations.A .ancilr-` hasfrf a f characteristic si: which is different from the amplitude-modulatedffl pulses rfrom the Udifferent:channels.: The synchronizing pulse AS is shown-@asta 'sin'gle.;pulse.l-f of greater` timeduration than 'any'.iofthefchan `65 l' nel pulses; 1 It willfthus:beiseenithat thesrepeti-aition rateof the fpulses'from;thelidifferent chan-f nel pulse generators lvand fromi :ther: synchroniz ing pulse lgeneratorfis thesame;.and1:.tha';t th channelfpulses are equally; spacedfapartfor eac frame or cycle.` of operationssr 1. Eac'lrrcycleof op' erationsor frame constitutesv a :synchro'nizin pulse followed bya `seriesmof.lspacedefcharme pulses extendingf overfzthehsintervalflfrom: hefl'be ginningf@ of.` oneesynchronizing.` p1'1lse'1to--A helfbe-f frequency? isgcaused toxvary: -froma l.mean value `ing signal) is 8 24+1)=200 kilocycles.

zietel...

giniiiziigoflfthef nextssynchronizingf.: pulsera The f duration of the pulses is made to..besho1xtaco paredl' to'theFitimecaintervals between them nso thsitfith'e differentirtransmissionss do mot coincidey Tlisllthedifferent apulsergeneratorsearek .x assignedisuccessiv meffpositionssiwithinf.ewhich t, they? l 'arel 'allo.vveclcfto'roper-atei.`

The .pulse#amplitude-modulatedfchannelvzisig-i nals and the@synchonizingepulsefe.inxcommomiz connection l3-modulate thefrequencylifofl oscillatonzrfl 1 l byifmeansi of=reactance1tube :modulators:

l 9: .Apparatusif Sand'zsl 'I farei locatednwithin' a dashedzaline :box/'i118 flabeledi-asubecarrienmod laters; .The afrequency of.'therlsuhecarrierrwave Stroms oscillator I 'lv atl-...outputs connection' f2 Iii i caused` .to:deviateiaboveifandzzbelowffse luy aI pea valueefdi .assis :shots/nainV Figi!A 7. l i, A".l'h'e'noutputof oscillator :'l.'l.yatu.lead:2 la is i thus;;a :,Wavefv .whosere amplitude` s1.: .substantially constant bute'whose in-accordance` With the.amp1itude ..andi-signfziof the'. pulseseiappearingon lead; :l 3.2 Iand constitu ing thenmodulationii Thef groups-of:frequencies, .i comprisingiaathe: output of therfsubecarrier :.oscllatorl andfifappearingsinzlead;.2I .modulateslf the f frequency of radici-.frequency oscillatoiu! 3f.which',x11. in turn; :is iconnectedfvia' R..'-:.l5.: transmissionrrlinen: 25 to the directive antenna 21.

In :a vtypical-.case involving .1241.voice.y :channels in a 'systenrtusingxthe presentsinvention; the'arefi petition .frate-x` fr' of .the lpulsesizin f zeach.; :channel may.;4 lcea .8` .kilocyclesi Las... .a Lresults' of.l which fthe` repetition.: rate. of; the.'.:pu-1ses.f:on iconnectional 3 (considering. thex24. channels.-.andathe;synchroniz- 'Ilieii;v duty '-.factorcDsof seachfpulsef should "bea as high aszpossible. f. If..;.D:=is J50: peri cent Jof y.thefintervalt .L betvvleenzadjacentpulsesf'onilead;` :I 3,2 thehmaxierr mum video 4: '(:DQ-Czspulse); y:componenti:appearing cycles.; The durationlofzfeaeh:pulse'maybelofff the order of .2.5 #secaj (microsecondarV `If thei de'f1` viation ratio .Riforithis component. is )set equal` 1f to :.unity, and; if the previous minimum valuekoi'f subfcarrier: lfrequencyiiof .2;5 fai vis assumed; then fsa-.411 megacycles. 1 The: second::modulations` at fit leadiA 25 may iinvolveza 'radio frejquency-tfrf.`=4000 megacycles. with La deviation `ratio'-'R2'=2 Thus S a final total .radioefrequency band-width-offfl Figi: 5 shows. one-1: embodiment*'ofl-the'r-PAM modulator `which' may Lbeused in the transmitter system .of Fig.1.2..In...FigI 5 Vthere is-shownfm-f sine. I wave.. oscillator l33 1 preferably -v crystalecmq;4 Y trolled) Whoseoutput--transformediinto theifform` of :.extrernely` short; pulses ..:feedsi-into -fandf locks 1r by injection tra short* pulse?` voscillator: 35 whose--zi frequency L or repetition -lratefis .the same-as that@ of fthe: sinewwave y-osci'llatcr:Iv 4The short outputNvt to 'la countenor-.step-voltage Wave'y generator@T.y The counter 31 provides-tworoutputsg'one of which Aisa step- `voltagewave .which vis supplied to lf'the Vcouplingamplifier `AVand the other of4 which is: a .synchronization 'pulse'v occurring vonce. l fon each step QV wavefcycle. 'andf which .is appliedl via lead.`39`ito a synchronizationpulseV generator 43. The functionof thesteplvoltage WavewhichwA is applied-.to theacoupling amplifier 41 -anda then tothe differentchannelsover-lead 4I is to timer the. occurrencev of- `'each channelvxpulsexrfi Lead 4i! ontaninga .theaaoutput: fof: thet-1 coupling. @amplia eriilll` fisfxcommon into ialliiof fthe" channelsf -1-;` 2 i'f ,ripeto fchannel n; rsi-Thus; alle channelsf=havei`-`1 their j inputs connected ,togethen in :electrically parallel relation.

All of the channels have substantially identical equipment and each includes inthe order named, a channel'selector 5l, a Vdifferentiating circuit 51, a self-restoring trigger circuit 59 functioning as a controlled pulse generator, a modulator 6| and a source of input waves 63 which may' be an audio-frequency signal source.r Y.

The channel selectors for the different channels are differently biased and each channel selector is Anormally biased well beyond thecurrent cut-off condition.. The bias of each channel selectorisso. adjusted that the applied step voltage Wave from the coupling amplier causesy current to flow, consecutively in the diierent channel, selectors.. One channel selector conducts for each rise -of voltage in thestep voltage wave. Each step rise in the step Wave is great enough to insure that during Iits occurrence the current of the corresponding biased channel se lector shall be driven rapidly from beyond the cut-off condition to a zero bias value'.A Once a channel selector starts to conduct, the current flow therein will continue until the end of the synchronization period, when the input' voltage to the'channel selectordrops to zero at the, end

ofthestepwave.

The circuits of apparatus 33, 35, 31, 41, and 5|, Ahave been described in great detail in copendingapplication Serial No. 608,957 iled August-4.19,45` by'W. .D. Houghton, now U. S. Patent 2,531,81'7.granted November 28, 1950, assigned to the common assignee, to. .which reference is made.

The outputfrom each channel selector 5I is in the form of a rectangular wave .pulsewhich ing to the leading edge of the output Wave from thechannel selector. The diierentiator circuit comprises; a. capacitor in circuit With a resistor.

The trigger circuit 59 may .comprise any suitable circuit having a stable state and an active state. By Way of example, such a trigger circuit may comprise two triode vacuum Vtubes'whose.

grids and anodes are interconnected regeneratively. One tube isnormally conducting and the other non-conducting inthe stable state, Vand vice versa in the active state.

it vto change from thestable to the active state in which it will remainfor aV time duration dependent upon the time constants of the trigger circuit elements, after which yit. restoresV itself to the stable state. The active period of the.

trigger circuit may, for a time duration of 2.5.csec. Y v A positivesquare pulse of 2.5 psedrduration and of a desired duty factor is obtained from the example, extend over trigger fcircuit when it is tripped and used to turn on normally non-conductive modulator cir- A tripping pulse will fire or activate the trigger circuit and cause z cuit 6|I for the 2.5 ',usec. duration. Modulator 6I is normally biased to the current cut-off condition, and it is turned on to full gain by the pulse from the trigger circuit. By way of example,

the modulator 6| may comprise a pair'of multigrid vacuum tubes invwhich the signals from trigger circuit 59 are applied to one pair of corresponding grids (the screen grids for example), inl parallelI whereas the audio frequency signal fromapparatus es isfappiied to thecorrespondf ing'controlgrids in push-pull. The result is a serlesof:pulse-amplitude-modulated signals in thepoutput leadof the modulator which includes positive and negative pulses of varying amplitudes.` Trigger circuit 59 can be considered as a switch to turn :on the modulator 6| which then' acts ,asxan amplifier for the audio frequencysignal-from source 63. The output from modulatorl isa pulse whose amplitude depends upon the instantaneous value of the signal source 53.V

The video pulse outputs from the modulators in all'thedifferent channels are combined in lead.61 and fed to common pulse amplifier 49. The output from common pulseamplier 49 combines with the video pulse output from the synchronizing pulse generator 43 to feed a common output connection I3 extending to the subcarrier modulator circuit.- The output in lead I3 .is shown in the curve of Fig. 6b.V The synchronizingpuls-e generator 43 produces a D.C. pulse at the end of each step voltage wave and this synchronizing pulseis of longer duration than the' channel pulses. s The synchronizing pulse mayhave aduration of 4 csec. compared tothechanneLpulses of 2.5 psec. each. This duty factor is. sufficient to minimize cross modulation. y g I It' will `thus be seen that all of the pulses in lead I3 are video (D.C.) pulses spaced from one another and occurring in a predetermined order or sequence.

The intermediate vrepeater station contains apparatus similar to that shown in Fig. 4. This apparatus. comprises 'a directive receiving antennall which is @pointed toward the next adjacent transmitter, whether it be another repeater station or a terminal transmitting station.. 'The receiving antenna 1| feeds a suitable -heterodyning receiver indicated by the dashed line box 8|,v containing therein any suitable radio-frequency amplifying apparatus, a mixeror converter 13,.a heterodyning oscillator 15, angintermediate-frequency amplifier 11 and an intermediate-frequency discriminator 19. The output from the intermediate discriminator 19. is in'the form of a sub-carrier wave constituting the modulation on the received wave, and this sub-carrier wave is fedto a sub-carrier amplifier |35'..` `The output energy from the subcarrier amplifier |35 is used to frequency-modulate a radio-frequency oscillator 23, contained within a dashed line box 28 labeled as a radiofrequencyA transmitter;V The output from the radio; frequency oscillator 23 is adoubly frequencymodulatedwavewhichis fed-via line 25 to a directional Vtransmitting vantenna 30 which is pointed toward the next station along the line of transmission. This next station may be another repeaterstation or the terminal receiving station. Y

Fig.- 3 shows one embodiment of the receiving terminal-station which includes a directional receiving antenna 1| feeding a radio-frequency receiver shown in the dashed line box BI. This receiver may include suitable radio-frequency amplifying apparatus, a frequency converter or mixer 13, .a local beating or heterodyning oscillator 15, an intermediate amplifier 11, and an intermediate discriminator 19 arranged in the mannerillustrated in the drawing. The output of the intermediatediscriminator 19 is fed via line 83to a subi-carrier demodulator shown in the dashed--linebox 89.. The electrical energy on lead @gamers 1F19 83 is in-the'iozxrrmcfl ay sub-carrierswave constitutingthe modulation-cn the received Waren-The fsub-carrierdemodulator9includes a sub-carrier -amplier followed by a sub-carrier discriminator 81. -`The-'energy derivedfromthe sub-carrier discriminator '81 appears onlead -9 I and isv -in the f form 'of a-series ofv amplitude-rnodulated-V pulses and a' synchronizing pulseof ftheform -shownin 6b. "These'pulses 'on lead-9|l ardorY video "(D.-C.) character -and are'fed to `the*PAMI delmodulator shown bythe dasl'ied-lineboxA 93. This `PAM demodulator -is illustrated in -rnore detail tand 'described later iii-*connection with ther-sys- "tem of Fig. 8, and performs the inversev of` the wfunctions of the PAM'modulator |-of"Figs 2 and 5. The outputs fromV the* PAM demodulator 93 "arein lthe form of" the original` signal waves at "the transmitter; for example, speech,` telegraph, Y Aor iiother suitable audio-frequency.signals origi- I nally; impressed at the-transmitter. Thesel signal "'Waves areindependent "oione Yanother andapg pear in the channels corresponding to the same channels atthe 'transmitter'.

. Referring to Figigthevideo pulses on vlead 9| f are impressed'vialead" 92 iupon a Vvcommon unit sh'own Within the dashed line box95. This com- 1 ,monunit includes asynchronizing pulsev separator circuit99 toJWhichthe video pulses are app plied,l a" pulse generator. `-I'Ill 'which is fed by the .output ofthexsynchronizing pulse, separator, a

j phasing trigger 'circuit |63,` asine-Wave exciter :1105; a limiter |91; astep-wave generator |99, and a couplingtube III, arrangedas indicated. The outputfrom the coupling rtube I II appears on vlead I`021and isyin" theform, of a 'step voltage L, Wave.

"'Intthejoperationfof the4 apparatus'in the common'unit 95,. thel synchronizing Vvpulse separator circuit iiilgserves` to produce a pulse corresponding 'jto the synchronizing pulse and suppresses lthe "channel pulses. V This "circuitp99'is able to do, this "'.because of the fact' thatfthe channel pulses are ofsh'orter duration thaniithe synchronizing pulse. '.`1The output4 from the synchronizing pulse separajtor99is a pulsel which is applied tothe pulse gen- 4 erator' I0 I This pulse generatorin'eiect is a dis- 1 chargepulse generator `which, produces a dis- Cgcharge pulse which is ,sent over two paths, one "'.of which extends tothe phasing trigger circuit "|03 and lthexzntherr of YWhich'cxtends via lead |94 to the" step-Wave generatorl I 99.

The'fphasingtrigger circuit |93 provides a pulse of adjustable `phaseforl driving; a sine-Wave exciter vacuum tuba circuit |95 having an output circuit tuned to a frequency corresponding `to the frequency "of the-sine-Wave4 oscillator 33 at the vxtermin'al transmitterl'Fig. 5. The output-of `this exciter I9 5v is,` fed to; the 'limiter AI 91 Awhich serves toconvert'" the applied sinewave to peaked posi- 'tive-pulses, incturn','appliedto the step Wave "-'generatorl |199.A Infpractice,' theV phasingy trigger circuit may, colmo-risevv two lvacuumtubes --suitably i coupled-'togethenwhile the* limiter Vmay also jcomprise a pair ,off vacuum tubes.

' LThef-step-Wave generator |99; comprisesl several y"vacuum tubes Whose function is' to produce a stepwavevoltage/having ar plurality 4 of` steps corresponding torthenumber` of `channels of thesystem. FThe' discharge pulse supplied from. the pulse generatorf-IUI via leadlIM to the step-Wave gen- --erator-IDB ser-ves `toiterrninate' the step-wave `voltmage 'after af-desired -number'lof rise-rs or steps. '5'I-he.1..output' fromiLthe step-Wave generator is .ctpassedathroughacoupling vacuum tube` I I If and :f'thenfiviatleadrl |32 tofthe variouszchannelf:Selectors ilo 'i vI-I 1 `*off-the diierent channelaunits;itfbeingrunder .-.stood that theres` are f .asimany lchannel..'units as l'there vare channels-.vinatheasystem; The diiferent channel units fed by energy in the lead IILIZW are :ishownwithin; afdashedgflinekboic 91. #The apparatus Withinijdashedilinerfboxiilrmay be called @the receivingmultiplex;channeli system.

:The Ychannel.selectors f I I I for;` the *i `different ,v channels are vdifferentlyzlciased.` sowas; to become conductive on different risers or .stepsroffthe applied step-wave voltage .iF-roirrrlead:- |102. alt will thus be seenzzthat ftfhefcommon,` unitc95 serves to separate`v the synchronizing: pulse ffromthe :channel-.pulses andto generate asuitable itmed voltage'Wavee-for 'energizing successively the channel lprising :thev AcommonZ unit-9 `The app.alfa'tushereinabove described4 as;` com- 5,;and51ch,annel `selecassignee The channelqselector` gII 'I is-sirnilar to The output from each channel selector ||`I iront;` `at ,i the` time that the particularr` channel l selec-tor ,vacuum4 tube-becomesfconductive-inresponse :to a -Dazrticular riseginfthez-,applied step Vol-tage Wave on lead |112. `wByf-rneans ofqdiierenr-tia-tor.circuit I'2I comprising a-.capacitorarld refthe-channeliselector after differentiationfby, cir

cuit IZI. The;edifferentiatormircuitI2|- andthe i. :trigger` circuiti 1235-arexsimilar to r4and roperate in `'the `same manner asgthe ,corresponding-i elements 51 and q 59in thetransmitting multiplex` f circuit i Fig; 5 previously` described.

@Whentriggerfcircuitl |523 `isf-tripped;it turns on rnormally non-conductive gatediamplier |25 4for a time-interval f onlyfsucienttto Vaccept :and

- pass;;channel. puiser# lllapplied f to; the Y. gated ampliergviaglead 2'I. vIta-.Should benoted that the inputs fof-,wallv gated:x ampliiiersein -the'diierent channel units lare connected', electrically in parallel to lead 9|. The gated amplierxI'ZS-mayfbe v a vacuumgtubepbiased:beyondthe current cut-off condition. 'Ihisfgated:amplifier'then Vrequires a rf suitable; pulsefoffLsuitablenpolarity (preferably a positivepulsey'rfromthe "triggeri circuit z 23 of i sufficient-magnitude utooverconfiesxthe cut-off bias n and cause'the :amplifier togxbecomef conductive.

Thisn pulse cof.` suitable-.polarity'- from: the trigger circuit 'f-If23gfmay be; napplied to@- the f signal grid, anotherrgrid -f or the cathode :of the` gated iamplier. f The time .constantsI of:` the triggerf circuit |23 ,i are so Chosen as-.to .cause the: gated 1 amplifier ato yaeceptfand pass-the pulses-.fori thatparticular s channel. vThe outputefromfth'e i gated ampliiier `for each channel-@unit is afpulse "of constant width andvaryingamplitudenfor-each frame or ---cycle0f operations,:as-illustrated :by the lchannel pulSesa-in-Figpb. These pulsesiofconstant `'Width lating frequencyfoathessignallwave. rThe output 1l of the low-pass filterk '|29 is thus the" audiofrequency for that channel and it may be amplified in A. F. amplifier l3l and fed to output line |33.

Although specific circuits have been described for achieving the results of the present invention, it should be understood that circuits different from those shovvnV in Figs. 5 and 8 may be used Without departing from the spirit and scope of the invention.

What is claimed is:

1. A multiplex communications system comprising a generator of D.C. pulses, means in each channel for modulatingV the amplitude of said pulses in accordance with the instantaneous values of 'samples of a signal Wave, to produce pulses whose amplitudes are proportional to and of the same algebraic sign as the instantaneous samples of the modulation Wave, an oscillator in common to said channels and producing oscillations of a frequency higher than the highest modulating frequency of said signal wave, means for modulating the frequency of said oscillator by the amplitude-modulated pulses, a radiofrequency oscillator, and means for modulating the frequency of said radio-frequency oscillator by the frequency-modulated output of said rst oscillator. Y y

2. A multiplex communications system having for each channel a source of audio Waves, a generator of spaced D.C. pulses occurring at a rate Vof Ythe order of 2.5 to three times the highest modulating frequency of said source, means for modulating the amplitudes of said pulses in accordance with the values of instantaneous samples of the audio Waves such that the resulting D.C. amplitude modulated pulses have an amplitude proportional to the instantaneous modulating audio Wave and of a predetermined algebraic sign relative to the sign of the instantaneous sample of the modulating wave; a first duced by said first oscillator, and means for modulating the frequency of said second oscillator by the frequency-modulated output of said first oscillator.

3. A pulse multiplex system comprising a plurality of channels each including a generator of time spaced D.C. pulses and means for modulating the amplitudes of said pulses in accordance wlth the values of instantaneous samples of a f signal wave, a common output circuit for the pulses from said channels, means for causing the pulses fromfsaid channels to occur sequentially in a predetermined order and in non-overlapping time relation,` a sub-carrier oscillator coupled to said common output circuit and producing a wave Whose mean frequency is higher than the highest modulating frequency of said channels, means for modulating the frequency of said subcarrier oscillator by the pulses in said common output circuit, a radio-frequency oscillator producing oscillations whose mean frequency is appreciably higher than the mean frequency of said sub-carrier oscillator, and means for modu- Y`latingV the frequency of said radio-frequency Y oscillator by the frequency-modulated output from said sub-carrier oscillator.

"4. In a pulse type multiplex communication. system, a plurality of channels each having a. normally non-conductive channel selector vacuum tube, a step-wave generator supplying the: inputs of the different channel selectors in parallel with a recurring step-voltage Wave having a plurality of steps of different voltage values,rsaid channel selectors being differently biased to become conductive in a predetermined order on different steps of the applied step-voltage wave, a generator of spaced D.C. pulses for each channel controlled by the channel selector in that channel, an amplitude modulator for each channel coupled to the output of and controlled by the pulse generator of that channel, and means for applying diierent signal modulating waves to the diiferent modulators, to produce in the output of each channel a series of separated D.C. pulses whose amplitudes vary in accordance with the values of the instantaneous samples of the modulating Wave and which occur at a rate higher than the highest frequency of the modulating Wave for that channel, and a common output circuit for said modulators.

5. In a pulse type multiplex communication system, a plurality of channels each having a normally non-conductive channel selector vacuum tube, a step-Wave generator supplying the inputs of the different channel selectors in parallel with a recurring step-voltage wave having a plurality of steps of different voltage values, said channel selectors being differently biased to become conductive in a predetermined order on different steps of the applied step voltage Wave, a pulse generator for each channel controlled by the channel selector in that channel for producing a series of spaced D.C. pulses, an amplitude modulator for each channel coupled to the output of and controlled by the pulse generator of that channel, and means for applying different signal modulating waves to the different modulators, whereby the output from each modulator is a pulse Whose amplitude is proportional to the instantaneous value of samples of signal wave for that channel, a common output circuit for said amplitude modulators, a sub-carrier oscillator coupled to said common output circuit through a reactance tube modulator, whereby the frequency of said oscillator is modulated by the pulses in said common output circuit, a radiofrequency oscillator, and means for modulating the frequency of said radio-frequency oscillator by the frequency-modulated output of said subcarrier oscillator.

6. In a pulse type multiplex communication system, a plurality of channels eachhaving a normally non-conductive channel selector vacuum tube, a step-Wave generator supplying the inputs of the different channel selectors in parallel with a recurring step-voltage Wave having a plurality of steps of different voltage values, said channel selectors being differently biased to become conductive in a predetermined order on different steps of the applied step voltage Wave, a self-restoring trigger circuit for each channel controlled by the channel selector in that channel and producing a D.C. pulse when tripped into the active state, an amplitude modulator for each channel normally biased to cut-off and controlled to become conductive by the pulse produced by the trigger circuit for that channel, and means for applying separate signal waves to the amplitude modulators of the different channels, to thereby produce in the output of each modulator a series of D.C. pulses of amplitudes varying in 'accordance with theinstantaneoussamples nelpulseiis variable ,over a'ffrange by v`theisignal modulation for.,that particular channel, aireceiving :systemA including Vapparatus for. producing D'.C;" pulses representative foff'rsaid channeltand 3synchronizing,pulses, means*y responsive :only to the,-D.-,C.2 synchronizing;` zpu'lse for producing a f f2 plurality,V fof substantiallycjequally spaced waves corresponding, in number; to :the -numberf of;4 channel and synchronizing pulsesiineach frame or cycle of operations, a step-Wave generator coupled to said means for-producing a step-volt-,fV i' -agerwave-having a -l plurality 'of stepsv` or frisers corresponding in number to the -nu'mber--'of=re ceived Waves for each frame or cycle of operations, a plurality of individual channel selector circuits coupled to the output of said step-wave generator, said channel selector circuits being so constructed and arranged as to become respon-V sive on different risers of the step voltagewave, a gating circuit for each receiving channel normally biased to cut-off, a trigger circuit for each channel arranged to turn on the gating circuit for that channel, means to control each trigger circuit from the output of the channel selector circuit for that channel, a connection for supplying D.C. channel pulses to all of said gating circuits in parallel, means for deriving from each gating circuit a pulse Wave of constant Width and varying amplitude corresponding to the channel pulse originally produced at the transmitter, and a low-pass lter for each channel for converting the varying amplitude pulses to the original signal modulating Wave for that channel.

8. A communications system comprising agenerator of pulses, a source of signals producing Waves having relatively positive and negative polarity portions comprising means for deriving vfrom said generator video pulses whose amplitude is proportional to and of the same algebraic sign as the instantaneous value of the Waves from said source, an oscillator producing oscillations whose frequency is higher than the highest modulating frequency of the waves from said source, means for modulating the frequency of said oscillator by said a-mplitude-modulated pulses, a radio-frequency oscillator and means for modulating the frequency of rsaid radio-frequency oscillator by the frequency-modulated output of said first oscillator.

9. In a pulse type multiplex communication system, a plurality of channels each having a normally non-conductive channel selector vacuum tube, said channel selectors being diierently biased, means for causing said channel selectors to become sequentially operative in a predetermined order, a generator of spaced D.-C. pulses for each channel controlled by the channel selector in that channel, an amplitude modulator for each channel coupled to the output of and controlled by the pulse generator of that channel, and means for applying different signal modulating waves tothe different modulators, to thereby produce in the output of each modulator a series of pulses of amplitudes varying in accordance with the instantaneous samples of the modulating wave applied to the respective moduz` ldixcarriersoscillator.

:coupled o,-said common :output E circuit :through a reactancezttube modulatorgpwhereby.'athe freaidseoscillator. is-` modulated i bythe :pulses .ff inrsa1d :common output "circuit, a :radio- '1;ireqnencyeoscillator; :and means :for sm'odulating fthe vfrequency 'fof fsaidfradio-frequency Voscillator f :by'i'zthee-frequencnemodulated',output of said `sub- OrzrIna. a; ipulsev @typen` multiplex "communication usystem, .a pluralityioffchannels eachhaving a normally non-conductive channel. selectouvacuum"..:.tube,:1I ..saidrichannel i selectors being differently biased, means for causing said channel selectors to--=become sequentially operative in a fpredetermined order, a--generatorof D.C. pulses for each channel controlled'fbyfthechannelf-selector infthat channel, an amplitude modulator for each channel coupled to' the output of and controlled by the pulse-generator of that' chanmodulating waves to the different modulators, to produce in the output of each channel a series of separated D.C. pulses whose amplitudes vary in accordance with the values of instantaneous samples of the modulating wave and which occur at a rate higher than the highest frequency of the modulating wave for that channel, and a common output circuit for said modulators.

ll. In a pulse type multiplex communication system, a plurality of channels each having a normally non-conductive channel selector vacuum tube, said channel selectors being differently biased, means for causing said channel selectors to become sequentially operative in a predetermined order, a generator of spaced D.C. pulses for each channel controlled by the channel selector in that channel, an amplitude modulator for each channel coupled to the output of and controlled by the pulse generator of that channel, and means for applying diierent signal modulating Waves to the diierent modulators, to thereby produce in the output of `each modulator a series of pulses of amplitudes varying in accordance with the instantaneous samples of the modulating wave applied to the respective modulator for that channel, a common output circuit for said modulators, a sub-carrier oscillator, means l coupled to said common output circuit and to said sub-carrier oscillator for modulating the frequency of said oscillator, in accordance with the pulses in said common output circuit, a radiofrequency oscillator producing oscillations of a frequency higher than that of the sub-carrier oscillator, and means for modulating the frequency of said radio frequency oscillator by the frequency-modulated output of said sub-carrier oscillator.

12. In a pulse type multiplex communication system, a plurality of channels each having a normally non-conductive channel selector vacuum tube, said channel selectors being differently biased, means for causing said channel selectors to become sequentially operative in a predetermined order, a differentiator circuit coupled to the output of l.each channel selector, a self-restoring trigger circuit coupled to and responsive to the output of each dilerentiator, and a modulator coupled to the output of each trigger circuit, and means for applying different signal modulating Waves to the different modulators, to thereby produce in the output of each modulator a series of pulses of amplitudes varying in accordance with the instantaneous samples S12-,sisma of the'm'odulatng wave applied to the-respective modulator for that channel, a common output circuit for said modulators, a sub-carrier oscillator, means coupled to said common output circuit and to said sub-carrier oscillator-for modulating the frequency of said oscillator in accordance with the pulses in said common output circuit, a radio-frequency oscillator producing oscillations of a frequency higher than that of the sub-carrier oscillator, and means for modulating the frequency of said radio frequency oscillator REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Name Date Number Miner Dec. 1, 1903 Number" Name Date -2,048,081 Riggs July 31, 1936 2,199,634 Koch May 7, 1940 2,233,183 Roder Feb. 25, 1941 2,262,838 Deloraine Nov. 18, 1941 2,282,046 Goldsmith May 5, 1942 V2,298,409 Peterson Oct. 13, 1942 2,311,021 Blumlein Feb. 16, 1943 2,358,382 Carlson Sept. 19, 1944 2,403,210 Butement July 2, 1946 2,407,308 Lorenzen Sept. 10, 1946 2,412,964 Chatterjea Dec. 24, 1946 2,413,440 Farrington Dec. 31, 1946 2,421,727 Thompson June 3, 1947 2,458,124 Wilmotte Jan. 4, 1949 2,476,162 Thompson July 12, 1949 2,480,137 Houghton Aug. 30, 1949 OTHER REFERENCES Multiplex Broadcasting, Grieg.

Electrical Communications, vol. 23, March 1946, pp. 19-26. 

